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A WIMP dark matter search with CsI(Tl) crystal

SeungCheon Kim

Department of Physics & Astronomy Seoul National University

Seminar@Fermilab (2012-07-31)

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Outline

A brief introduction about dark matter

A Description about KIMS experiment

A study about the background

The recent results

The prospect

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Dark matter ?

Hubble image of coma cluster

1936 Zwicky first proposed the existence of dark matter, inferred from the motion of galaxies in coma cluster.

=> The total mass of the luminous matter is too small to explain the velocity of the galaxy in the cluster!

107-108KWhat is holding such hot gases ?

The recent x-ray measurement of the cluster also requires the invisible matter!

4Big Bang Nucleosynthesis

Gravitational Lensing Rotation curve for Galaxy

Bullet clustercosmic microwave

background

A pile of evidences for dark matter !!!

99 % of people believe the dark matter…

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What we know about dark matter

Dark (invisible) Þ no EM interaction rarely interacting very stable

MassiveÞ gravitationally evident non-relativistic (cold – most, not necessarily all of them) Influencing the structure formation of the Universe

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What we know about dark matter

Overwhelmingly abundantÞ the majority of the matter constituents in the universe ExoticÞ Not made of the known matter Need of the new theory other than SM (multifunctioning particle – not invented for DM – favored)

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WIMP (Weakly Interacting Massive Particle)- a good dark matter candidate

Ω : energy density of the universeΩ = ΩDM + ΩΛ + Ωbaryon = 1 for the flat universeΩDM = 0.222 ± 0.026 from WMAP

The weakly interacting relic particles can survivethe annihilation and so exist in this level of the density.

Supersymmetry and Univ-ersal extra dimensiontheories provide good candidates of this kind.Þ lightest neutralino lightest kluza-klein particle

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WIMP (Weakly Interacting Massive Particle)- a good dark matter candidate

And, it is testable with the experiment, even though very challenging.

It has been expected WIMP will deposit a few tens keVin the detector absorber.

The event rate will be very small.

Background reduction & discrimination is essential.

σ χ-nu-

cleon

=10-4 pb

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KIMS (Korea Invisible Mass Search)

A WIMP search experiment using CsI(Tl) scintillators

It has been carried out at Yangyang underground lab(Y2L),South Korea.

Seoul National University: H.C.Bhang, J.H.Choi, K.W.Kim, S.C.Kim, S.K.Kim, J.H.Lee, J.I.Lee , J.K.Lee, M.J.Lee, S.J.Lee, J.Li, X.Li, S.S.Myung,S.L.Olsen, I.S.SeongSejong University: U.G.Kang, Y.D.KimKyungpook National University: H.J.Kim, J.H.So, S.C.YangYonsei University: M.J.Hwang, Y.J.KwonEwha Womans University: I.S.HahnKorea Research Institute of Standard Sciences:Y.H.Kim, K.B.Lee, M. LeeTsinghua University : Y.Li, Q.Yue, J. Li

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CsI (Tl) scintillator ?

Widely used, well-known detectorEasy handling

Large scintillation yield : 60,000/MeV

technique

nuclear recoil

electron recoilLow hygroscopicity

Pulse Shape DiscriminationÞ Distinguishing nuclear recoil event at the statistical basis

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CsI (Tl) scintillator ?

Large atomic number, Cs (133), I (127) Good for A2 scaling

High spin expectation value for proton Sensitive to SD interaction

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CsI (Tl) scintillator ?

However, it has some inherent background.

137Cs artificial radioisotope , half-life of 30 yearsbeta decay & delayed (2.5min) gamma of 662 keV

134CsProduced by the capture of neutron from the cosmicray muon, half-life of 2 years, beta decay & severalprompt gamma rays

87RbIt exists in the pollucite (ore for Cs).Natural abundance of 27.84 %, beta decay

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CsI (Tl) scintillator ?

It contains iodine (I) like DAMA/LIBRA experiment.Þ Cross-checking the WIMP-I interaction scenario for DAMA positive signal.

So, it’s very good idea to search for WIMP usingCsI (Tl) scintillators if one can manage its inter-nal background .

One detector module : one CsI Crystal + 2 PMTs

PMT : 3” PMT (9269QA),

RbCs photo cathode (green extended)

Crystal size: 8x8x30 cm3 (8.7 kg)(Beijing Hamamatsu Photon Techniques Inc.)Background level: 2-3 counts/keV/kg/day (cpd)

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Am241 calibra-tion

13.9keV Np L X-ray17.8keV Np L X-ray20.8keV Np L X-ray26.35keV gammaCs, I X –ray escape59.54 gamma...

Light yield:5-6 p.e/keV

Eth = 3 keV

- data- simulation (GEANT4.9.5)

CsI (Tl) detector for KIMS

Event window is 40µs.

Digitized with 400MHz FADC

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Detector array

3 x 4 detector arrayThe total mass : 103.4 kg

Multiple hit eventsÞ Vetoed for DM search But, references for calibration & event selection

E of det10

Total E except det10

134Cs

134Ba

4+3+

4+

2+

2+

0+

1969.871643.281400.551167.93

604.70

0.0

E [keV]

β-

27.3%

2.50%70.1%0.033%

0.10%

Decay for 134Cs (T1/2=2.06yr)

Measurement: 2.053±0.145yr

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Neutron shield & Muon Veto(30 cm mineral oil)

Lead shield (15 cm)

Polyethylene(5 cm)

Copper shield(10 cm)

12 x CsI (Tl) crystal

N2 gas flow inside the Cu shield

KIMS Detector system

Yangyang Pumped Storage Power Plant

Experimental site:Yangyang Underground Laboratory (Y2L)

Minimum depth : 700 m / Access to the lab by car (~2km)

(Upper Dam)

(Lower Dam)

(Power Plant)

KIMS (DM Search)AMoRE (Double Beta Decay Experiment)

SeoulY2L

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The Experimental hall

Muon flux : 2.7 x 10-7/cm2/s (2000 m.w.e)

Neutron flux : 8 ×10−7/cm2/s for 1.5-6 MeV neutronsRn background in the air : 1-2 pCi/l

Clean room systemOnline-Monitoring of temperature, humidity, power stability, Rn background level

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Background study

Multiple hit only

α spectrum

Crystal : 134,137Cs, 87RbPMT : 238U, 232Th, 40KSurface : progenies of 222Rn

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Background study

pmt0pmt1

Surface alpha (SA) background

Scintillation at PMT body

Clean acryl boxPMT PMT

These events makes troublein pulse shape discrimination.SA => Characterize its PDFPMT events Þ Find cuts to minimize its contamination.

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γ & β back-ground(electron recoil (ER) events)-main background

Background level (cpd) & their origin

Internal source ~ 1 (134,137Cs) ~ 0.1 (238U, 232Th)~ 0.3 ( 87Rb)

PMT ~ 0.5 (238U, 232Th, 40K)

Crystal surface (progenies of 222Rn)

0.1 – 1 (?) (mainly, 210Pb)

Background at the low energy

A few tens of recoil energy will be measured as a few keVbecause quenching.The current background level below 10 keV : 2.8 counts/keV/kg/day (cpd) + PMT dark current

Surface alpha (SA)

~ 0.1

Scintillation at PMT body

< 0.1

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Rejection of events produced from PMT itself=> PMT body scintillation & dark current (PMT events)

PMT generates events by itself.

Þ PMT body scintillation + After pulse

+ random coincidence + etc.

=> It can be problematic for the low energy, rare phe-nomena search.

After pulse

Þ A delayed (~1 μs) pulse following the primary pulse

Þ The atom in the PMT vacuum ionized by the accelerat-ing photo-electron drifts to the photo-

cathode producing a big signal (spike-like).

Signal size=> # of SPE

Time span btw neighboring SPEs(us)

pmt0 pmt1

Afterpulses in PMT Dummy Detector

Afterpulse by postive ion(H,He,N2 ...) in PMT

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Clean acryl boxPMT PMT

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PMT events rejection

Compton events

From the PMT dummy detector data,

We have developed the event selection condition to

minimize the PMT events contamination.

The Efficiency for nuclear recoil (NR) events (WIMP candidate) : ~30%

tail events

Clean acryl boxPMT PMT

Pulse Shape Discrimination (PSD)

Discriminating the event type by the pulse shapeEnabling the estimation of NR event rates statistically

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, LMT10=logi ie

i

Att t

A

PSD parameter

Reference data for PSDSurface alpha (SA) : from the Radon-contaminated crystalNuclear Recoil (NR) : by irradiating the test crys-tal with Am-Be neutron source Electron Recoil (ER) : compton scattering

SA

NR

ER

at 3 keV

Opening the data…

Period: September 2009 – August 2010 => 317.15 live daysExposure: 24524.3 kg•days

After applying the PMT noise selection cut,Assuming the data are composed of SA, ER and NR eventsEstimating the fraction NR event rates with PSD.

Candidate events for WIMP : NR occurring in one detector

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The estimation of the NR events rate

Pdf = f0 x FNR + f1 x FSA + (1-f0-f1) x Fgamma

The posterior pdf for f0 & f1 is obtainedfrom Bayesian analysis method.

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The estimated NR event rates for 12 detectors

90 % limit68 % interval

SA

The positive signals at higher energy bins seem cor-related with SA contamination.

The PSD PDF of NR at higher energy bins might be contaminated with gamma ray caused by the inelasticscattering. => need of further study

For the conservative claim, in the combining all the detectors,the detectors with high SA contamination are ex-cluded.

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The overall NR events rate

det0, 8, 11 excludedTotal exposure: 24524.3 kg days

No meaningful excess of NR events is observed.

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KIMS NR event rates & DAMA/LIBRA annual modulation amplitude

The annual modulation amplitude from DAMAÞ At 2-4keV, 0.0183±0.0022 counts/day/kg/keV

Þ At the corresponding energy range (3.6-5.8 keV) in KIMS, (QF for DAMA =0.08 adopted) The 90 % C.L limit is 0.0098 counts/day/kg/keV This is well below the DAMA modulation amplitude. Any scenario with Iodine for DAMA is not consistent with KIMS results !!!

DAMA/LIBRA The spectrum of the amplitude of the annual modulation

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KIMS & DAMA in iDM model

For 70 GeV mass WIMP

CsI

Expected spectrum for iDM for I & Cs target=> M=70 GeV, δ=116keV, vesc=500 km/s, ve=235km/s, σ=0.93e-3 pb

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The new exclusion limits from KIMS

Spin-Independent interaction Spin-dependent proton interaction

Published in PRL 108, 181301 (2012)

Annual modulation study in KIMS

The analysis of 2.5 years data is almost completed.PSD was not taken into account. There’s time-varying background from 134Cs (2year half-life).The results will be reported very soon.

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Sep. 2009 ~ Feb. 2012

3-6 keV

Prospect of KIMS experiments

New PMTs are under test.Þ ~10 p.e/keV ? Decreasing the energy thresholdÞ Reduced 238U, 232Th, 40K Decreasing current background level by ~ 1 cpd

The treatment of the surface backgroundÞ Polishing or surrounding with active veto layer ?

Adding low background NaI (Tl) scintillators is be-ing planed.

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Summary

12 CsI (Tl) scintillators (103.4kg) have beenrunning at Yangyang underground lab.

Most of backgrounds of the detector has been under-stood.

The improved limit of WIMP-nucleon interaction hasbeen derived based on 1 year data with PSD.

Annual modulation study based on 2.5 years was almost done and will be reported very soon.

New upgrade is being planed.

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Back up

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Event rate of WIMP in the detector

number of target nuclei

number density of WIMP

WIMP nucleon cross section

Coherent enhancement along nucleus

nuclear form factor

The effect by the velocity Distribution of WIMP

If σn = 10-5 pb, for 100 GeV WIMP and Iodine target,The expected event rate is 1-2 counts/keV/kg/day,considering quenching.

CsI (Tl) detector for KIMS

One detector module : one CsI Crystal + 2 PMTs

PMT : 3” PMT (9269QA), Electron tube

RbCs photo cathode

(green extended)

Crystal size: 8x8x30 cm3 (8.7 kg) (Beijing Hamamatsu Photon Techniques Inc.)Background level: 2-3 counts/keV/kg/day (cpd)

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Am241 calibra-tion

13.9keV Np L X-ray17.8keV Np L X-ray20.8keV Np L X-ray26.35keV gammaCs, I X –ray escape59.54 gamma

5-6 p.e /keV

Neutron shield(30cm mineral oil)

Lead shield (15cm)

Polyethylene(5cm)

Copper (10cm)

CsI(Tl) ScintillatorNeutron detectorMuon detector(Neutron sheild)

KIMS Detector system

N2 gas flow inside the Cu shield

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Annual Modulation Studies (w/o PSD)

Sep. 2009 ~ Feb. 2012

Total DAQ rate is under 6Hz .

2.5 year data to see annual modulation ; 75.53 ton∙days

The temperature of detector array is 20 - 21.6 OC depending on the position, and it is maintained stably with a maximum fluctuation of around 0.2 OC.

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3-6 keV

0

0 0

2cos ( ), 2.980 , t 153(June 2)

365

t t

peak peakR N e b A t t y

Annual modulation amplitude is ob-tained including the exponential de-cay of 134Cs.

Annual modulation amplitude is consistent with null.

The 90% upper limit of the ampli-tude is comparable to DAMA’s an-nual modulation signal (0.0189 cpd/kg/keV)

We are trying to give final numbers shortly in this summer.